An accurate determination of the path of a device across a surface is important in a variety of applications. For example, in the field of optical scanners, there must be accurate information as to the travel of the scanning device along the original so that a faithful representation of an image of a scanned original may be acquired. Typically, the captured image provided by a scanner is a pixel data array that is stored in memory in a digital format. A distortion-free image requires a faithful mapping of the original image to the pixel data array.
Information as to the travel of a scanning device along an original may be obtained by an optical navigation system comprising a photo-receiver array. Optical navigation systems utilizing primarily analog-based technology are known in the art. For example, optical navigation systems comprising a photo-receiver array, an analog spatial image enhancement filter, and an analog correlator with off-chip bias and control are known. While usable, the accuracy of such analog systems is not optimal since so much of the algorithmic processing of the image signal (e.g., filtration, correlation) is done in the analog domain. The complementary metal oxide silicon (CMOS) technology supporting chips performing this processing cause slight imperfections and variations in the physical CMOS structure (e.g., non-linearity, device mismatches, power supply issues), to substantially affect a predictable and repeatable performance of such analog systems. As a result, analog optical navigation systems are not very suitable for mass manufacture.
Digital optical navigation systems, such as the one disclosed in U.S. patent application Ser. No. 09/040,640 of Badyal et al. filed Mar. 18, 1998, entitled "CMOS Digital Optical Navigation Chip", have therefore been developed to overcome the disadvantages of analog optical navigation systems. Badyal discloses a CMOS digital integrated circuit (IC) chip on which an image is captured, digitized, and then processed on-chip in substantially the digital domain. A preferred embodiment of the IC disclosed by Badyal comprises imaging circuitry including a photo-receiver array for capturing an image and a charge transfer stage for generating a representative analog signal, conversion circuitry including an n-bit successive approximation register (SAR) analog-to-digital converter for converting the analog signal to a corresponding digital signal, filter circuitry including a spatial filter for edge and contrast enhancement of the corresponding image, compression circuitry for reducing the digital signal storage needs, correlation circuitry for processing the digital signal to generate result surface on which a minima resides representing a best fit image displacement between the captured image and previous images, interpolation circuitry for mapping the result surface into x-and y-coordinates, and an interface with a device using the chip, such as a hand-held scanner. The filter circuitry, the compression circuitry, the correlation circuitry and the interpolation circuitry are all advantageously embodied in an on-chip digital signal processor (DSP). The DSP embodiment allows precise algorithmic processing of the digitized signal with almost infinite hold time, depending on storage capability. The corresponding mathematical computations are thus no longer subject to the vagaries of a CMOS chip structure processing analog signals. As a result, precise and accurate navigation enables a predictable, reliable and manufacturable design. Parameters may also be programmed into the DSP's "software," making the chip tunable, as well as flexible and adaptable for different applications.
A preferred embodiment of a photo-receiver in the photo-receiver array disclosed by Badyal et al. is further described in U.S. Pat. No. 5,769,384 of Baumgartner et al. issued June 23, 1998, entitled "Low Differential Light Level Photoreceptors", and in U.S. patent application Ser. No. 09/024,092 of Knee et al. filed Feb. 17, 1998, now U.S. Pat. No. 6,104,020, entitled "Electronic Shutter for a Low Differential Light Level Photo-Receiver Cell". Operation of the photo-receivers described in Baumgartner et al. and Knee et al. is further described in U.S. Pat. No. 5,149,980 of Ertel et al. issued Sep. 22, 1992, entitled "Substrate Advance Measurement System Using Cross-correlation of Light Sensor Array Signals".
The above disclosed patents of Baumgartner et al. and Ertel et al., as well as the patent applications of Badyal et al. and Knee et al., are hereby incorporated by reference for all that they disclose.